Engineered Clostridium botulinum toxin adapted to deliver molecules into selected cells

ABSTRACT

An engineered payload-delivery system includes a target cell binding unit, covalently bound to a pore forming unit, and a payload portion adapted with a region capable of non-covalently binding to the pore forming unit. The pore forming unit is derived from a particular sub-serotype of Clostridium toxin, while the payload region is derived from a different sub-serotype of Clostridium toxin. The disclosed chimeric protein-based composition is capable of specifically delivering payload to neural cells.

RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application No. 62/162,582, filed May 15, 2015, which is incorporated by reference into the present application in its entirety and for all purposes.

GOVERNMENT RIGHTS

This invention was made with government support under grant number HDTRA-10-C-0055 awarded by Defense Threat Reduction Agency of the Department of Defense. The government has certain rights in the invention.

BACKGROUND

Clostridium botulinum, a spore-forming, heat-resistant, anaerobic bacterium, produces a protein-based toxin (botulinum toxin) having several serotypes, known as A through G serotypes, of which serotype C has 3 subtypes, known as serotypes C1, C2 and C3. The C1 neurotoxin paralyzes people and animals in low doses by blocking acetylcholine release by neurons, recovery is slow-treatment may require ventilation for multiple weeks before a person is able to breathe again. The C2 toxin is not a neuro-active, and causes necrosis and hemorrhaging. The C3 toxin is the least characterized of the C sub-serotypes.

Most toxin-based delivery systems are multi-domain proteins that bind target cells and translocate material (payloads) across the lipid bilayer into the cytosol of the targeted cell. These systems are altered AB-type toxins, consisting of a payload domain (A) and a binding/translocation domain (B). The A and B domains can be covalently linked by a polypeptide or disulfide bond that is later cleaved during the translocation step. Non-covalently linked (binary) A and B toxin domains are transcribed and translated independently and associate prior to exerting toxicity. The Clostridium botulinum C2 toxin (C2) is not a neurotoxin, but it has a binary AB toxin design.

SUMMARY

The present disclosure advances the art by providing chimeric toxin-based delivery compositions (or systems) for delivering payload (or agent) to a target cell. In one embodiment, the composition may contain a target cell binding unit, a pore-forming unit and a payload unit, with or without other additional components.

In one embodiment, the pore-forming unit may be the same as the pore-forming unit of known toxins, for example, toxins from Clostridium botulinum. In another embodiment, the pore-forming unit may be derived from the pore-forming unit of known toxins with modifications. In another embodiment, the pore-forming unit may be any proteins that may function as a pore-forming unit.

The payload unit may contain the agent to be delivered to the target cell. In one aspect, the payload unit may bind non-covalently to the pore forming unit, or to the linked pore forming unit and target cell binding unit. In another aspect, the pore forming unit and target cell binding unit are linked covalently.

In one embodiment, the payload unit may be the same as the payload unit of known toxins, for example, toxins from Clostridium botulinum. In another embodiment, the payload unit may be derived from the payload unit of known toxins with modifications. In another embodiment, the payload unit may be any proteins that may function as a payload unit for the delivery of the agent.

In another embodiment, the target cell binding unit may contain a specific target cell binding ligand selected from the group consisting of antibody, antibody fragment, affibody, growth factor, a receptor-binding ligand, or combinations thereof. In another embodiment, the target cell binding unit may contain a native or modified binding domain derived from a heavy chain of C. botulinum toxin other than C2.

In another embodiment, the target cell binding unit preferentially binds to a neural cell. In another embodiment, the composition preferentially delivers the agent to neural cells. In another embodiment, the target cell binding unit is the binding domain of the heavy chain of C. botulinum neurotoxin C1 (C1 Hcc) (See FIG. 2c , corresponding to amino acids 230-426 of SEQ ID NO: 1).

In another embodiment, the pore-forming unit may be a polypeptide derived from a first type of non-neurotoxic (i.e., not specifically targeting neuron) toxin, and the target cell binding unit may be a polypeptide derived from a second type of toxin, wherein the second type is different from the first type of toxin. In another embodiment, the first type of non-neurotoxic toxin may be a binary toxin.

In another embodiment, the pore-forming unit may be a polypeptide derived from a first Clostridium toxin sub-serotype, while the target cell binding unit is a polypeptide derived from a second Clostridium toxin sub-serotype, wherein the second sub-serotype is different from the first sub-serotype. In another embodiment, the pore-forming unit is a polypeptide derived from the pore-forming unit of Clostridium botulinum toxin C2 (See FIG. 2c , corresponding to amino acids 1-591 of SEQ ID NO: 2).

In another embodiment, the pore-forming unit may contain a native or modified domain derived from a toxin other than C. botulinum toxin C2. In one aspect, the pore-forming unit may contain a native or modified pore-forming domain derived from a toxin selected from the group consisting of Clostridium perfringens alpha-, beta-, epsilon- and iota-toxin, Clostridium spiroforme Iota-like toxin, anthrax toxin, and combinations thereof.

In one embodiment, the pore-forming unit is a polypeptide derived from the pore-forming unit of Clostridium botulinum toxin C2, while the target cell binding unit may contain a native or modified binding domain derived from a toxin other than C. botulinum toxin C2. In one aspect, the target cell binding unit may contain a native or modified binding domain derived from a toxin selected from the group consisting of C. botulinum neurotoxins, Clostridium perfringens toxins alpha, beta, epsilon and iota toxin, Clostridium spiroforme Iota-like toxin, cholera toxin, anthrax toxin, shiga toxin, shiga-like toxin, diphtheria toxin, ricin, exotoxin A, and combinations thereof.

In another embodiment, the payload unit is not covalently bound to the target cell binding unit or the pore-forming unit. In another embodiment, the payload unit is a polypeptide derived from Clostridium botulinum toxin C2 (See FIG. 2d , SEQ ID NO: 3).

In another embodiment, the agent comprises at least one member selected from the group consisting of a therapeutic agent, a diagnostic agent, an imaging agent, and combinations thereof. In another aspect, the agent may contain at least one member selected from the group consisting of a toxin, a cell cycle blocker, an apoptosis inducing agent, an inhibitor of DNA replication, an inhibitor of RNA synthesis, an inhibitor of protein synthesis, an enzyme, a protein binding agent, an antibody, a neutralizing antibody, a labeling agent, magnetic beads, and combinations thereof.

In another embodiment, the agent comprises an ADP-ribosyltransferase. In another embodiment, the agent comprises C2I from Clostridium botulinum toxin C-2. In another embodiment, the agent comprises a fluorescent agent for labeling or monitoring the target cell.

In one embodiment, the target cell may be a cancer cell. In another embodiment, the target cell may be a neuron. In another embodiment, the target cell may be a cell of a brain tumor, a cell of a neuroblastoma, a cell of a retinoblastoma, a peripheral neuron, a motor neuron, a sensory neuron, or combinations thereof.

In another embodiment, the engineered payload-delivery composition may include a target-cell binding unit that is covalently bound to a pore-forming unit, and a payload portion adapted with a region capable of non-covalently binding to the pore forming unit. In another embodiment, a polypeptide (SEQ ID NO: 4) is disclosed which may contain the target-cell binding unit covalently bound to the pore-forming unit linked by a linker, (EP)₁₀, wherein the target-cell binding unit is the binding domain of the heavy chain of C. botulinum neurotoxin C1 (C1 Hcc) and the pore-forming unit is a polypeptide derived from the pore-forming unit of Clostridium botulinum toxin C2.

In another embodiment, the active payload region is bound to the pore-forming unit through a coupling region derived from the light-chain payload portion of botulinum toxin C-2. In another aspect, the target-cell binding unit is derived from the target-cell binding unit of botulinum toxin C-1.

In another embodiment, the target cell binding unit is the binding domain from the heavy chain of C. botulinum neurotoxin C1 (C1 Hcc), while the pore-forming unit is the pore-forming unit of Clostridium botulinum toxin C-2, and the payload unit comprises C2I from Clostridium botulinum toxin C-2.

In another embodiment, in a composition for delivering an agent to a target cell comprising a target cell binding unit, a pore-forming unit and a payload unit, the target cell binding unit comprises a polypeptide having at least 80, 90, 95, 99%, or 100% sequence identity to the amino acid sequence 230-426 of SEQ ID NO: 1, and the pore-forming unit comprises a polypeptide having at least 80, 90, 95, 99%, or 100% sequence identity to amino acid sequence 1-591 of SEQ ID NO: 2, and the payload unit comprises a polypeptide having at least 80, 90, 95, 99%, or 100% sequence identity to amino acid sequence of SEQ ID NO: 3.

In another embodiment, in a composition for delivering an agent to a target cell comprising a target cell binding unit, a pore-forming unit and a payload unit, the target cell binding unit and the pore-forming unit are covalently linked to form a polypeptide having at least 80, 90, 95, 99%, or 100% sequence identity to the amino acid sequence of SEQ ID NO: 4, and the payload unit comprises a polypeptide having at least 80, 90, 95, 99%, or 100% sequence identity to the amino acid sequence of SEQ ID NO: 3.

In another embodiment, the disclosed composition may be administered to a subject by injection, wherein the subject contains the target cell(s).

In another embodiment, a polynucleotide encoding a polypeptide is disclosed, wherein the polypeptide has at least 80, 90, 95, 99%, or 100% identity to a polypeptide selected from the group consisting of SEQ ID NOs 1-6. In one aspect, the polynucleotide may be carried on a vector. In one aspect, the vector may be capable of replicating itself.

In another embodiment, a host cell comprising the polynucleotide is also disclosed. The host cell may be used to produce the composition for delivering the agent to a target cell. In another embodiment, the host cell may be introduced into a subject for delivery of the agent. In another embodiment, for purpose of this disclosure, the host cell may be a bacterium, or a virus.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows (a) Molecular steps of intoxication by the native C2 toxin; and (b) Model of neural delivery based upon the C2II-C1 and C2It transport system.

FIG. 2 shows protein domains of C. botulinum C1, C. botulinum C2II, fusion C2II-C1, and C2I. Numbers correspond to amino acid residues of each protein. (a) BoNT C1 has a linked enzymatic payload domain (light chain) and a binding/translocation domain (heavy chain). (b) The C2II binding/translocation component has four domains. Amino acid residue 182 indicates the trypsin cleavage position for activation of C2II into C2IIa. Domain 4 (D4) was removed to produce C2IIΔD4 as the translocation domain for C2II-C1. (c) The fusion C2II-C1 (SEQ ID NO: 4) was made by linking C2IIΔD4 (amino acids 1-591 of SEQ ID NO: 2) and BoNT C1 H_(CC) (amino acids 230-426 of SEQ ID NO: 1) with an (EP)₁₀ linker flanked by glycine-serine residue pairs on both sides (SEQ ID NO: 5). Amino acid 182 is the activation site for C2II-C1. (d) The native C2I enzymatic payload of the C2 toxin and truncated C2It domain. Amino acids 299, 348, 387 and 389 are essential for ADP-ribosylation activity of C2I (SEQ ID NO: 3) and are therefore not present in C2It (SEQ ID NO: 6).

FIG. 3 shows Flow cytometry to evaluate C2II-C1-mediated uptake of C2It-488 to differentially GT1b-enriched cell populations. (a) Coomassie stained SDS-PAGE of purified C2It (˜26 kDa). Lanes: M: molecular ruler, 1: soluble elution fraction after thrombin cleavage. (b) Indicated N2A cells were GT1b-enriched and subsequently incubated with recombinant proteins activated C2II-C1 (2 μg/mL) and C2It-488 (4 μg/mL) for 2 hours. Cells were then processed with pronase (1 μg/mL) to remove membrane-bound C2It-488. Samples were analyzed by a BD FACS Canto II flow cytometer using FACSDiva software. (c) Quantitative assessment of intracellular fluorescence by flow cytometry. Percentages are expressed as a mean±SEM (n=3) and statistical significance of GT1b-dependent uptake of C2It-488 mediated by C2II-C1 was calculated using Student's t-test by comparison to each control mean value. p<0.005.

FIGS. 4 (a) and (b) show CLSM images of GT1b differentially GT1b-enriched N2A cells treated with C2II-C1 and C2I-568. All images were captured with 60× oil lens with 2× optical zoom. N2A populations were treated with a Rab5a-GFP early endosome marker (green) for 24 hr and stained with DAPI. Activated C2II-C1 (2 μg/mL) C2It-568 (red, 4 μg/mL) and GT1b (50 μg/mL) were incubated for 2 hours. (a) Cells were enriched with GT1b for 4 hr prior to addition of proteins. (b) Cells were not GT1b-enriched prior to addition of proteins.

FIG. 5 shows Cell rounding of differentially GT1b enriched cell populations by C2I mediated by C2II-C1. (a) Coomassie stained SDS-PAGE of purified C2I. Expected masses: C2I-GST (˜75 kDa), C2I (49 kDa) molecular ruler, 1: lysis supernatant, 2: purification resin prior to thrombin cleavage, 3: soluble elution fraction after thrombin cleavage. (b) A172 glioblastoma cells were grown to ˜60% confluence and enriched as indicated with or without GT1b. (c) Flow cytometry of synchronized HeLa cells stained with propidium iodide with and without release from thymidine block over time was used to confirm progression of S phase DNA synthesis after removal of excess thymidine and addition of deoxycytidine for release. (d) Cell rounding of differentially GT1b-enriched synchronized HeLa cells.

DETAILED DESCRIPTION

Compositions and methods for delivering molecular payloads to the cytosol of target cells are disclosed. Bacteria have evolved mechanisms to target cells and deliver toxic payloads to the cytosol of target cells. This mechanism may be modified and engineered to deliver beneficial payloads.

In general, there are two classes of AB-type bacterial toxins: linked and unlinked (binary). Linked toxins typically have a single chain protein containing both a toxin domain and a binding/translocation domain. Binary toxins typically have two separately expressed protein molecules, where the binding/translocation domain and the toxin domain assemble via non-covalent interactions.

For purpose of this disclosure, the term “derived” means a molecule is constructed based on another molecule and is identical, substantially identical or substantially similar in structure to that other molecule. In another aspect, the derived molecule typically performs identical, substantially identical or substantially similar functionality as the other molecule.

The term sequence identity is used to denote the similarity in amino acid or nucleotide sequence. Where a smaller molecule is compared to a larger molecule, the smaller molecule may be compared to the full-length or a partial fragment of the larger molecule.

The native C2 toxin is composed of two separate proteins. The B domain protein (C2II) binds target cells and translocates the A domain (C2I, the payload). The A domain is an ADP-ribosyltransferase that causes cell rounding and apoptosis initiated by ADP-ribosylation of cytoplasmic actin (FIG. 1a ). C2II monomers are proteolytically processed to remove a 20 kDa segment from the N-terminus, which activates the binding/translocation domain into C2IIa. C2IIa monomers then spontaneously oligomerize and bind the cell surface via interactions with asparagine-linked glycans on the cell membrane. The A domain, C2I, binds to the C2IIa oligomers and the C2IIa/C2I complex is internalized by clathrin and Rho-dependent mechanisms. Acidification of the early endosome causes membrane pore formation by C2IIa oligomers, through which C2I is transported into the cytoplasm.

For therapeutic development, engineering of a binary toxin has certain advantages because the binding/translocation domain and the payload domain may be separately expressed and purified. The C2 toxin from C. botulinum is a binary structure, but is nonspecific as it binds a variety of cells and necessitates N-linked glycans for intoxication (i.e., it is not a specific neurotoxin). Disclosed here are methods to engineer the C2 toxin binding domain by retargeting to neural cells. More specifically, the target binding domain from the C1 botulinum neurotoxin may be used. The binding domain from the C1 botulinum neurotoxin has been previously applied as a targeting component for drug delivery to peripheral neural tissue in linked toxin designs and as liposomal surface modifications.

In one embodiment, binding domain replacement of the C2 toxin requires that the retargeted binding/translocation component retain its ability to oligomerize upon activation, bind to the new targeting moiety on the cell surface, and translocate the payload into the cytosol of the target cell. The natural binding domain of the C2 toxin is located at the C-terminal end of the molecule and is designated as D4 (see FIG. 2b ). In one aspect, D4 is not required for oligomerization because translocation pores can be formed in artificial membranes even when D4 is absent. In another aspect, D4 is deleted from C2II and replaced with the BoNT C1 binding domain that would target the molecule to peripheral neurons. The BoNT C1 Hcc (FIG. 2a ) preferentially binds gangliosides GT1b and GD1b.

In another embodiment, BoNT/A N-terminal heavy chain domain (HCN) is not included in the chimeric C2II-C1. It has been shown that HCN may assist in the orientation of the toxin for association with the membrane by interacting with phosphatidylinositol phosphates. It is shown that that although HCN may be active in native BoNT translocation, it is not required in a chimeric C2II-C1 translocation event.

In another embodiment, the binding domain is taken from a linked toxin and inserted into the binding/translocation domain of a binary toxin. This configuration retarget the resulting molecule to neurons while maintaining the C2 toxin's mechanism of activation and translocation. It should be noted that a similarity exists between BoNT and C2 endocytosis and translocation mechanisms in that a clathrin/rho/dynamin-mediated endocytic-endosomal entry pathway characterized by pH-dependent protein conformational changes is implicated for both toxins.

In another embodiment, attempts have been made to express a soluble C2II-C1 fusion protein that would oligomerize when activated with trypsin. Direct fusion of the C1 H_(CC) domain was not successful due to solubility problems. To remedy this limitation a flexible glycine-serine linker (G₄S)_(n) was used but encountered similar issues. Finally, use of a rigid (EP)₁₀ linker resulted in a soluble fusion protein that is compatible with activation and oligomerization. SDS-PAGE confirmed that the C2II-C1 fusion protein could be activated by limited trypsin digestion and then oligomerize. Western blotting is used to confirm that the C1 H_(CC) domain is incorporated into the oligomeric species. BoNT C1 antigenicity specific to the C2II-C1 oligomer and a decrease in electrophoretic mobility in comparison to C2IIΔD4 demonstrate that C1 H_(CC) at the C-terminus of C2II-C1 does not prevent oligomerization and is compatible with limited trypsin digestion.

To quantify and visualize binding and internalization of a payload by C2II-C1, a fluorescently labeled C-terminally truncated C2I-based payload, C2It (FIG. 2d ), was constructed for use in flow cytometry and microscopy experiments. C2It that was composed of amino acids 1-226 of C2I (not containing the ADP-ribosylating active site residues), was fluorescently labeled in two separate versions with Alexa Fluor 488 (C2It-488) and 568 (C2It-568) by amine reactive chemistry. Previously, BoNT C1 H_(C) entry was shown to be GT1b-dependent in N2A cells that were artificially enriched for GT1b³⁰, and this strategy was adapted to study targeting by the C2II-C1 fusion protein. If the engineered B component, C2II-C1, were activated, oligomerized and associated with the fluorescently labeled A component, C2It, GT1b-dependent uptake of fluorescently labeled C2It should be observed. This cellular model does not employ electrostimulation as previously described to enhance BoNT C1 intoxication because entry alone was presumed to be sufficient for the non-neural-specific C2 component of the fusion to promote translocation activity. For flow cytometry, a culture of N2A cells was enriched with GT1b while another was not, both cultures were incubated with activated C2II-C1 and C2It-488, and then both cultures were treated with pronase to remove extracellular proteins prior being analyzed. Cells with intracellular fluorescence above 10³ absorbance units were counted by flow cytometry and repeated results showed that N2A cell populations enriched with the binding domain receptor GT1b preferentially took up C2II-C1-delivered fluorescent C2It (FIG. 3). The results shown here indicate that the BoNT C1 H_(CC) can be used to replace another toxin binding domain and result in a GT1b-dependent entry specificity. To confirm this uptake is dependent on GT1b and to determine subcellular localization within N2A cells, confocal microscopy is employed (FIG. 4). C2It-568 preferentially enters GT1b-enriched cells and does not colocalize with fluorescently labeled early endosomes. Escape from the early endosome by transport of C2It through the pore created by the translocation domain is a determinant of payload delivery to the cytosol. These results are consistent with the expected association between the engineered payload and binding/translocation domain by GT1b-specific delivery of C2It by C2II-C1. Lack of colocalization of early endosomes with C2It-568 (FIG. 4(a)) provides evidence to pursue other payloads with the intent of cytosolic delivery to manipulate the cytosome.

To deliver an active enzyme to the cytosol by the C2II-C1 fusion, the native C2 toxin A component, C2I may be produced. The C2I enzyme is known to cause cell rounding in eukaryotic cells by ADP-ribosylation of cytosolic actin. The effect of C2I is tested after delivery by C2II-C1 to human glioblastoma A172 and HeLa cell lines that are enriched with the ganglioside GT1b. A greater than two-fold increase in cell rounding of GT1b-enriched cell populations is found for both cell lines when compared to controls lacking GT1b enrichment. By comparison, payload-induced cell rounding of synchronized HeLa cells in the presence of the fusion translocator C2II-C1 is less efficient than reported by Barth et al. in the presence of the native C2II translocation domain. A truncated form of C2II-C1 characterized during expression may have incorporated into C2II-C1 oligomers, which may result in a decrease in binding efficiency. Although an apparent lack of monomeric C2II-C1 in final purification fractions is evident by SDS-PAGE, it is possible that monomeric C2II-C1 dissociated or not incorporated into oligomers compete for binding with the functional form of the oligomeric delivery system. These findings confirm the native cytosolic activity of the C2I enzyme specifically delivered by C2II-C1 in a GT1b-dependent manner.

In another embodiment, alternate payloads based on modified C2It may be used in delivery applications of the C2II-C1 fusion protein affecting the natural targets of BoNTs (FIG. 1b ). A minimal region of amino acid residues 1-87 in the C2I component is required for complementary activity with the native C2II translocation domain. Translocation of non-canonical polypeptides may also be possible with modified C2I, similar to payload development work recently conducted with anthrax lethal factor. This disclosure provides the basis of exploring other binding specificities and payload domains for additional applications.

The following examples are provided to illustrate the present disclosure, but are not intended to be limiting. The chemicals and physical parameters are presented as typical reagents or parameters, and various substitutions or modifications may be made in view of this disclosure by one of skills in the art without departing from the principle and spirit of the present invention.

EXAMPLES Example 1 Construction and Expression of Chimeric Constructs: C2II-C1, C1 HCC, C2ΔD4, C2It and C2I

Plasmid pUC57-C2II-C1 HCC was purchased as a codon-optimized gene synthesis product. It consists of the C2II gene truncated by seven C-terminal amino acids upstream to the C1 HCC sequence, representing BoNT C1 amino acids Y1094-E1291. Primers C2IIΔD4F and C2IIΔD4-GS(EP)R amplified the gene corresponding to C2II amino acids M1-T592 and added a 5′ BamHI extension and 3′ glycine-serine-(EP) linking region to be used for overlapping PCR with the C1 H_(CC) domain. The BoNT C1 H_(CC) gene was PCR amplified with primers (EP)GS-C1 H_(CC)F and C1 H_(CC)R to contain a 3′ EcoRI restriction site. A second round of PCR was performed using GS(EP)₁₀GSF and C1 H_(CC)R to extend the 5′ amplicon of the C1 H_(CC) to complement the 3′ of the C2IIΔD4-GS(EP) sequence. The two resulting fragments were fused by overlapping PCR to yield C2IIΔD4-GS(EP)₁₀GS-C1 H_(CC) (C2II-C1). To generate C1 H_(CC), PCR amplification was performed on the pUC57-C2II-C1 H_(CC) template using primers C1 H_(CC)F and C1 H_(CC)R. To generate C2IIΔD4, primers C2ΔD4F and C2ΔD4R were used to amplify the C2II gene without domain 4. Plasmid pUC57-C2It, was purchased as a codon optimized gene synthesis product. C2It (corresponding to C2I amino acids 1-226, PDB 2J3V) was directly subcloned into pGex-2T using BamHI and EcoRI restriction sites. Full length C2I (corresponding to C2I amino acids 1-431) was generated by overlapping PCR by fusion of C2It to DNA amplified from a synthetic DNA using C2IF and C2IR as flanking primers and C2IOF and C2IOR as overlapping primers. All final PCR products were digested by BamHI and EcoRI and ligated into pGex-2T. DH5α was transformed by electroporation to propagate C2II-C1, C1 H_(CC), C2IIΔD4, C2It and C2I as N-terminal GST fusions. DNA construct identities were confirmed with sequencing. Primer sequences are listed in Table 1.

TABLE 1 Primer sequences C2IIΔD4F CGCGGATCCATGCTGGTCTCC (SEQ ID NO: 7) C2IIΔD4- CCGGCTCTGGTTCCGGTTCAGAACCGGTGATCACTTT GS(EP)R GACCA GAATATTCATG (SEQ ID NO: 8) (EP)GSC1 CCAGAACCAGAGCCAGAACCAGGTTCTACCAACGTTG H_(CC)F TCAAA GACT ATTGGGG (SEQ ID NO: 9) C1 H_(CC)R CGGGAATTCTTATTCTGAAACCGGGAC (SEQ ID NO: 10) GS(EP)₁₀GSF AACCGGAACCAGAGCCGGAACCGGAACCGGAACCGG AGCCA GAACCAGAGCCAGAACC (SEQ ID NO: 11) C1 H_(CC)F CGCGGATCCATGGGCACCAACGTTGTCAAAGACTAT TGG (SEQ ID NO: 12) C2IIΔD4R CGGGAATTCTTA GGTGATCACTTTGACCAG (SEQ ID NO: 13) C2IF CGCGGATCCATGCCGATTATTAAAGAACCGATTGACT TCATC AACAAACCGG (SEQ ID NO: 14) C2IR CCGGAATTCTTAGATTTCTTTGTTTTGGATACCTTCAG CATCA AT (SEQ ID NO: 15) C2IOF GCAAGAACTGGACTTTTACAACAAAGGCTCGGAAGCCT GGGG TGCGGAAAACTATG (SEQ ID NO: 16) C2IOR CATAGTTTTCCGCACCCCAGGCTTCCGAGCCTTTGTTG TAAAA GTCCAGTTCTTGC (SEQ ID NO: 17)

Fusion proteins were overproduced in E. coli BL21 (DE3). All cell lines were grown in 400 mL LB, 100 μg/mL ampicillin at 37° C. until induction at OD₆₀₀˜0.5 with 0.5 mM IPTG at 25° C. for 16 hr. Cells were harvested in 100 mL aliquots and the pellets were stored at −20° C. Cells were resuspended in PBS, 1% Triton, pH 7.4, and a French press was used to lyse aliquoted cells by three passes at 10,000 psi. Cell debris was removed by ultracentrifugation at 80,000×g for 20 minutes at 4° C. Immobilized glutathione agarose (Genscript) was used to affinity purify GST fusion protein supernatants in batches using 150 μL of washed resin per 15 mL of culture supernatant and an incubation time of 1 hr at 4° C. Resin was washed with PBS pH 7.4 to remove unbound protein. Proteins were cleaved from the GST tag according to manufacturer's recommendations by bovine thrombin and separated from the purification resin by filtration using glass wool in a syringe. C2II-C1 was further processed by incubation with trypsin for 30 mins at a 1:5 enzyme to substrate ratio concluding with trypsin deactivation by trypsin inhibitor as described to activate recombinant C2II.

C2II-C1, C2IIΔD4, C1 H_(CC), C2I and C2It were separated by SDS-PAGE using a 10% polyacrylamide gel or by a 4-12% gradient Bis-Tris Gel. An anti-BoNT C1 polyclonal antibody (Metabiologics Inc., Madison, Wis.) was used to identify C2II-C1 using purified C1 H_(CC) as a positive control and C2IIΔD4 as a negative control. Proteins were separated by SDS-PAGE, transferred to a nitrocellulose membrane in Towbin buffer, blocked with 5% powdered milk in PBS-tween buffer and then probed with a 1:5,000 dilution of a 1 μg/ul anti-BoNT C1 antibody in 0.5% powdered milk in PBS-tween. Anti-rabbit HRP secondary antibody in 0.5% powdered milk, PBS-tween (1:5,000), was used for signal detection with ECL blotting substrate.

Neuro-2a cells (N2A) (ATCC, CCL-131) were cultured in Eagle's minimal essential medium (EMEM) supplemented with 10% (v/v) fetal bovine serum (FBS) and penicillin-streptomycin. A172 cells were grown in DMEM supplemented with 10% (v/v) FBS and penicillin-streptomycin (100 U/mL-100 μg/mL). HeLa cells (ATCC, CCL-2) were cultured in EMEM supplemented with 10% FBS and penicillin-streptomycin. HeLa cells were synchronized by double thymidine block with deoxycytidine release prior to ganglioside enrichment. Ganglioside-enriched cells were prepared by sonicating 50 μg/mL GT1b (Enzo Life Sciences, Farmingdale, N.Y.) in low-serum (0.5% FBS) culture medium for 20 min at room temperature. Cells were subsequently incubated 4 hr with GT1b. Prior to addition of recombinant proteins, cells were washed three times with PBS to remove free ganglioside from the culture medium. Flow cytometry with a 488 nm laser line and 586/42 bandpass filter on a BD FACSCanto II was used to confirm HeLa synchronization by staining of DNA with propidium iodide. 10,000 cells/events were counted and statistical significance of average fluorescence per cell was determined by Student's t-test (n=3).

Amine reactive Alexa Fluor dyes were dissolved in anhydrous DMSO (10 mg/mL) and stored as aliquots at −20° C. Purified proteins were concentrated to >5 mg/mL and adjusted to pH 8.5-9.0 with addition of 1 M sodium bicarbonate. Alexa Fluor in anhydrous DMSO was added to protein solutions with continuous stiffing for 1 hr at room temperature. Excess Alexa Fluor and DMSO was removed by gel filtration (G-25 resin). Labeled proteins were ultra-centrifuged at 80,000×g and subsequently assessed for degree of labeling by spectrophotometry before and after ultracentrifugation. A degree of labeling greater than 1 fluorescent molecule per molecule of protein was used as a quality control cutoff and there was no visible pellet or appreciable change in spectrophotometric qualities after ultracentrifugation.

N2A cells were grown in 24 well culture plates to ˜80% confluence. Cells were enriched with GT1b as indicated in FIG. 3c . Activated C2II-C1 was added at 4 μg/mL and C2It-488 at 2 μg/mL using a 0.5 mL working volume and incubated with cells for 2 hours. Cells were washed twice with PBS, then trypsinized and harvested. Cells were centrifuged and resuspended in PBS with pronase (1 μg/mL) and incubated on ice for 5 minutes. Protease inhibitor cocktail was then added and cells were centrifuged and resuspended in PBS with inhibitor cocktail. 10,000 events/cells were then counted by BD FACS Canto II flow cytometer using a 488 laser line and 530/30 emission band-pass filter. C2It-488 positive cells (greater than the absorbance threshold 10³ absorbance units) were counted and evaluated as a percentage of total cells. Replicated experiments were evaluated by Student's t-test (n=3).

Collagen-coated 12 mm no. 1 coverslips were placed into 24-well culture plates and seeded with N2A cells. N2A cells were grown to ˜80% confluence. Purified C2It was labeled with Alexa Fluor 568 succinimidyl ester (C2It-568) instead of Alexa Fluor 488 to allow for discrimination from the early endosome marker. The baculovirus transduction system, BacMam 2.0 Cell Lights Rab5a-GFP early endosomal marker (Life Technologies), was added ˜24 hr prior to GT1b enrichment. Cells were then enriched with GT1b as described in our methods. Recombinant proteins were added after washing of cells to remove free gangliosides. Activated C2II-C1 was added at 4 μg/mL and C2It-568 at 2 μg/mL using a 0.5 mL working volume and incubated with cells for 2 hours. Cells were washed with PBS, fixed with 4% paraformaldehyde and stained with DAPI. After processing, an Olympus Inverted IX-81 Microscope was used with an Olympus FV 500 confocal laser scanning microscope in sequence mode with laser lines 405 nm (blue), 488 nm (green) and 543 nm (red) to capture fluorescence images. Corresponding emission barriers used were 430-460 nm, 505-550 nm and 560-610 nm respectively. Transmitted light was used for cell morphology, and all images were captured using a 60× oil lens with 2× optical zoom. Contrast of all images was increased by 20%. Human glioblastoma A172 cells (ATCC, CRL-1620) were grown in 24 well culture plates to ˜60% confluence to reduce cell rounding observed at higher confluence. HeLa cells were synchronized as described in the previous section. Both cell lines were enriched with GT1b as described in our methods at 50 μg/mL. C2II-C1 was added at 40 μg/mL and C2I was added at 20 μg/mL using a 0.5 mL working volume and incubated with cells for 7 hours. Pictures of cells were taken using an Amscope IN300TC inverted stereo microscope at 40× using Amscope MT v 3.0.0.5 soft-ware. Rounded cells were counted and determined as a percentage of total cells in the frame. The experiment was replicated three times and evaluated for statistical significance with Student's t-test (n=3). Institutional biosafety committee approval was obtained prior to execution of experiments with C2I in a biosafety level 2 laboratory due to anticipated toxicity when combined with C2II-C1.

Example 2 Retargeting the Clostridium botulinum C2 Toxin to Neuronal Cytosol

Multiple recombinant protein constructs were expressed and purified using E. coli that were based on the BoNT sub-serotype C1 neurotoxin and the ADP-ribosylating C2 toxin. The native BoNT C1 is depicted in FIG. 2a , and the native C2II binding/translocation domain is depicted in FIG. 2b . C2 toxin with C-terminal deletion of domain 4 (C2IIΔD4) and the C1 neurotoxin binding domain C1 H_(CC) were produced for use as controls. The C2IIΔD4 and C1 H_(CC) of BoNT C1 (1094-1291) were linked with a glutamate-proline ten-repeat peptide linker (EP)₁₀ to generate C2II-C1 (FIG. 2c ). In addition, two C2I-based payloads were constructed including a non-toxic C2It (1-226) that excludes the active enzyme site and a full length C2I (1-431) (FIG. 2d ).

Cleavage of the glutathione affinity tag (GST) and activation of C2II-C1 by trypsin into oligomers was confirmed. E. coli BL21(DE3) cells were lysed and ultracentrifuged to remove insoluble proteins and the supernatant was passed over the affinity resin. The resin was then washed extensively, and protein-bound resin was loaded to examine the mass of the full length resin-bound protein and the extent of thrombin cleavage. The resin was then treated with thrombin to cleave the GST tag. Proteins were then eluted from the resin and treated with trypsin. The trypsin-activated C2II-C1 monomers oligomerized as indicated by a shift in electrophoretic migration from an observed mass of −90 kDa to a much greater mass than 250 kDa. Activated C2IIΔD4 was also produced with the same method and compared to activated C2II-C1. The heptameric form of C2II-C1 had an expected molecular mass of ˜497 kDa, and heptameric C2ΔD4 had as an expected molecular mass of ˜350 kDa. The C2II-C1 oligomer had a higher mass than that of C2IIΔD4 oligomer, as expected. The oligomerized forms of C2II-C1 and C2ΔD4 maintained stability in SDS during electrophoresis and dissociated partially with the addition of heating. An additional band was identified during purification with anti-BoNT C1 antigenicity. However, after extensive heating of C2II-C1 oligomers, it was determined the dissociated composition was predominantly of full-length C2II-C1 monomers.

Western blotting was conducted of oligomerized C2II-C1 and C2IIΔD4 Proteins were then probed with an anti-BoNT C1 antibody. BoNT C1 HCC (MW ˜23 kDa) was used as a positive control. C2II-C1 oligomers cross-reacted with the anti-BoNT C1 antibody, while the oligomerized C2IIΔD4 did not cross-react. This confirmed that BoNT C1 HCC was successfully fused to C2IIΔD4 via the (EP)10 repeat linker in the oligomeric state.

Neural Targeting of Fluorescently Labeled C2It Payloads by C2II-C1.

The binding and payload internalization that were mediated by the C2II-C1 binding/translocation component was investigated using a fluorescently labeled C2I-based payload, C2It (FIG. 2d ), to populations of cells with and without the GT1b ganglioside receptor. Murine neuroblastoma Neuro-2A (N2A) cells do not naturally present GT1b on the cell surface, but can be artificially enriched. After payload C2It was purified (FIG. 3a ), an Alexa Fluor 488 succinimidyl ester label was conjugated to the protein (C2It-488). C2It and activated C2II-C1 were incubated with differentially GT1b-enriched N2A cells. After removal of extracellular proteins by enzymatic digestion, flow cytometry was used to quantitate internalized C2It-488. The highest number of cells with increased fluorescence corresponded to GT1b enrichment and addition of C2II-C1 (FIG. 3b panel 2). Background uptake of C2It without C2II-C1 in the presence and absence of GT1b was minimal. Enrichment alone with GT1b did not give significantly increased background fluorescence (FIG. 3b panels 1, 4). Uptake of the C2It-488 payload alone was also minimal (FIG. 3b panels 3, 6). The highest non-target uptake (˜2%) occurred in the population with C2II-C1 and C2It without GT1b (FIG. 3b panel 5). Student's t-test determined that the dependence of C2It-488 uptake on C2II-C1 and GT1b was significant with p-values <0.05 between experiments. Overall, an intracellular C2It-488 delivery efficiency of ˜18% (percentage of cell population) was achieved in the presence of GT1b and C2II-C1 (FIG. 3c ).

After quantitation of binding and internalization by flow cytometry, C2It delivered by activated C2II-C1 to targeted cells was visualized by confocal fluorescence light microscopy to determine intracellular localization. C2It was conjugated to an Alexa Fluor 568 fluorescent dye (C2It-568). Channel separated imaging was conducted of C2It-568 (red), Rab5a-GFP early endosomal marker (green), and DAPI nuclei (blue). It was observed that an intracellular C2II-C1-delivered C2It-568 colocalized at a low level with early endosomes when cells were enriched with GT1b (FIG. 4(a)). This result was consistent with endosomal escape of C2It by an active translocation domain. Without GT1b, C2It-568 signals were confined generally to the outside of the cell with low levels of reporter associated with early endosomes (FIG. 4(b)). These findings are consistent with the binding/internalization flow cytometry data (FIG. 3b,c ). Additional control permutations lacking C2II-C1, GT1b or C2It-568 did not achieve intracellular delivery of C2It reporters with early endosomal dissociation.

Retargeting of the Native C2I Enzyme by C2II-C1.

Delivery of an active enzyme to the cytosol was determined by cell rounding caused by native C2I payload in both human glioblastoma A172 and synchronized HeLa cell lines differentially enriched with GT1b. Full length C2I was purified (FIG. 5a ), combined with activated C2II-C1, and then added to cell line cultures for seven hours. Cell rounding was determined to be 2.8-fold higher than the non-GT1b enriched A172 cell population (FIG. 5b ). Delivery-dependent cell rounding of synchronized HeLa cells was investigated as a non-neural cell line enriched with GT1b for a comparison to previous data of the wild type C2II without GT1b enrichment previously reported by Barth et al. Infect. Immun. 67, 5083-5090 (1999). Flow cytometry methods confirmed the synchronization of HeLa cells in the early S-phase by quantitation of DNA (FIG. 5c ). In synchronized HeLa cells, rounding was 2.1-fold above the non-GT1b enriched population. (FIG. 5d ). Student's t-test was used to evaluate the experimental significance between experiments. Comparing control populations to the GT1b-enriched population produced p-values <0.05.

List of Sequences of SEQ ID NOs 1-6:

List of sequences of SEQ ID NOs 1-6: SEQ ID NO: 1 - NNINDSKILSLQNRKNTLVDTSGYNAEVSEEGDVQLNPIFPFDFKLGSSG EDRGKVIVTQNENIVYNSMYESFSISFWIRINKWVSNLPGYTIIDSVKNN SGWSIGIISNFLVFTLKQNEDSEQSINFSYDISNNAPGYNKWFFVTVTNN MMGNMKIYINGKLIDTIKVKELTGINFSKTITFEINKIPDTGLITSDSDN INMWIRDFYIFAKELDGKDINILFNSLQYTNVVKDYWGNDLRYNKEYYMV NIDYLNRYMYANSRQIVFNTRRNNNDFNEGYKIIIKRIRGNTNDTRVRGG DILYFDMTINNKAYNLFMKNETMYADNHSTEDIYAIGLREQTKDINDNII FQIQPMNNTYYYASQIFKSNFNGENISGICSIGTYRFRLGGDWYRHNYLV PTVKQGNYASLLESTSTHWGFVPVSE SEQ ID NO: 2 - MLVSKFENSVKNSNKNYFTINGLMGYYFENDFFNLNIISPTLDGNLTFSK EDINSILGNKIIKSARWIGLIKPSITGEYILSTNSPNCRVELNGEIFNLS LNTSNTVNLIQGNVYDIRIEQLMSENQLLKNYEGIKLYWETSDIIKEIIP SEVLLKPNYSNTNEKSKFIPNNTLFSNAKLKANANRDTDRDGIPDEWEIN GYTVMNQKAVAWDDKFAANGYKKYVSNPFKPCTANDPYTDFEKVSGQIDP SVSMVARDPMISAYPIVGVQMERLVVSKSETITGDSTKSMSKSTSHSSTN INTVGAEVSGSLQLAGGIFPVFSMSASANYSHTWQNTSTVDDTTGESFSQ GLSINTAESAYINPNIRYYNTGTAPVYNVTPTTTIVIDKQSVATIKGQES LIGDYLNPGGTYPIIGEPPMALNTMDQFSSRLIPINYNQLKSIDNGGTVM LSTSQFTGNFAKYNSNGNLVTDGNNWGPYLGTIKSTTASLTLSLPDQTTQ VAVVAPNFSDPEDKTPRLTLEQALVKAFRLEKKNGKFYFHGMEISANQKI QVFLDRNTNVDFENQLKNTANKDIMNCIIKRNMNILVKVITFKENISSIN IINDTNFGVESMTGLSKRIKGNDGIYRASTKSFSFKSKEIKYPEGFYRMR FVIQSYEPFTCNFKLFNNLIYSNSFDIGYYDEFFYFYCNGSKSFFDISCD IINSINRLSGVFLI SEQ ID NO: 3 - MPIIKEPIDFINKPESEAKEWGKEEEKRWFTKLNNLEEVAVNQLKNKEYK TKIDNFSTDILFSSLTAIEIMKEDENQNLFDVERIREALLKNTLDRDAIG YVNFTPKELGINFSIRDVELDRDISDETLDKVRQQIINQEYTKFSFISLG LNDNSINESVPVIVKTRVPTTFDYGVLNDKETVSLLLNQGFSIIPESAII TTIKGKDYILIEGSLSQELDFYNKGSEAWGAENYGDYISKLSHEQLGALE GYLHSDYKAINSYLRNNRVPNNDELNKKIELISSALSVKPIPQTLIAYRR VDGIPFDLPSDFSFDKKENGEIIADKQKLNEFIDKWTGKEIENLSFSSTS LKSTPSSFSKSRFIFRLRLSEGAIGAFIYGFSGFQDEQEILLNKNSTFKI FRITPITSIINRVTKMTQVVIDAEGIQNKEI SEQ ID NO: 4 - MLVSKFENSVKNSNKNYFTINGLMGYYFENDFFNLNIISPTLDGNLTFSK EDINSILGNKIIKSARWIGLIKPSITGEYILSTNSPNCRVELNGEIFNLS LNTSNTVNLIQGNVYDIRIEQLMSENQLLKNYEGIKLYWETSDIIKEIIP SEVLLKPNYSNTNEKSKFIPNNTLFSNAKLKANANRDTDRDGIPDEWEIN GYTVMNQKAVAWDDKFAANGYKKYVSNPFKPCTANDPYTDFEKVSGQIDP SVSMVARDPMISAYPIVGVQMERLVVSKSETITGDSTKSMSKSTSHSSTN INTVGAEVSGSLQLAGGIFPVFSMSASANYSHTWQNTSTVDDTTGESFSQ GLSINTAESAYINPNIRYYNTGTAPVYNVTPTTTIVIDKQSVATIKGQES LIGDYLNPGGTYPIIGEPPMALNTMDQFSSRLIPINYNQLKSIDNGGTVM LSTSQFTGNFAKYNSNGNLVTDGNNWGPYLGTIKSTTASLTLSLPDQTTQ VAVVAPNFSDPEDKTPRLTLEQALVKAFRLEKKNGKFYFHGMEISANQKI QVFLDRNTNVDFENQLKNTANKDIMNCIIKRNMNILVKVITGSEPEPEPE PEPEPEPEPEPEPGSTNVVKDYWGNDLRYNKEYYMVNIDYLNRYMYANSR QIVFNTRRNNNDFNEGYKIIIKRIRGNTNDTRVRGGDILYFDMTINNKAY NLFMKNETMYADNHSTEDIYAIGLREQTKDINDNIIFQIQPMNNTYYYAS QIFKSNFNGENISGICSIGTYRFRLGGDWYRHNYLVPTVKQGNYASLLES TSTHWGFVPVSE SEQ ID NO: 5 - GSEPEPEPEPEPEPEPEPEPEPGS SEQ ID NO: 6 - MPIIKEPIDFINKPESEAKEWGKEEEKRWFTKLNNLEEVAVNQLKNKEYK TKIDNFSTDILFSSLTAIEIMKEDENQNLFDVERIREALLKNTLDRDAIG YVNFTPKELGINFSIRDVELDRDISDETLDKVRQQIINQEYTKFSFISLG LNDNSINESVPVIVKTRVPTTFDYGVLNDKETVSLLLNQGFSIIPESAII TTIKGKDYILIEGSLSQELDFYNKG

While the disclosure has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the disclosure. It is to be understood that various changes may be made in adapting the invention to different embodiments without departing from the inventive concepts disclosed herein and comprehended by the claims that follow.

REFERENCES

The contents of all cited references (including literature references, patents, patent applications, and websites) that may be cited throughout this application or listed below are hereby expressly incorporated by reference in their entirety for any purpose into the present disclosure. The disclosure may employ, unless otherwise indicated, conventional techniques of immunology, molecular biology and cell biology, which are well known in the art.

The present disclosure also incorporates by reference in their entirety techniques and methods well known in the field of molecular biology. These techniques include, but are not limited to, techniques described in the following publications.

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What is claimed is:
 1. A composition for delivering an agent to a target cell, comprising a target cell binding unit, a pore-forming unit, a rigid synthetic linker linking the target cell binding unit and the pore-forming unit, and a payload unit comprising the agent, wherein said payload unit binds non-covalently to a pore formed by the pore-forming unit upon activation and oligomerization of the pore-forming unit; wherein the pore-forming unit is a polypeptide derived from a first AB type of non-neurotoxic binary toxin, and wherein the target cell binding unit is a polypeptide derived from a second AB type of toxin, said second AB type being different from the first AB type, wherein the target cell binding unit binds to a neural cell, and wherein the target cell binding unit and the pore-forming unit are covalently linked through the rigid synthetic linker to form a polypeptide having at least 90% sequence identity to the amino acid sequence of SEQ ID NO:
 4. 2. The composition of claim 1, wherein the pore-forming unit is a polypeptide or a polypeptide oligomer derived from the pore-forming unit of Clostridium botulinum toxin C2.
 3. The composition of claim 1, wherein the composition delivers the agent to a neural cell.
 4. The composition of claim 1, wherein the target cell binding unit is a polypeptide or a polypeptide oligomer derived from the binding domain of the heavy chain of C. botulinum neurotoxin C1 (C1 Hcc).
 5. The composition of claim 1, wherein the agent comprises at least one member selected from the group consisting of a therapeutic agent, a diagnostic agent, and combinations thereof.
 6. The composition of claim 1, wherein the agent comprises at least one member selected from the group consisting of a toxin, a cell cycle blocker, an apoptosis inducing agent, an inhibitor of DNA replication, an inhibitor of RNA synthesis, an inhibitor of protein synthesis, an enzyme, a protein binding agent, an antibody, a neutralizing antibody, a labeling agent, magnetic beads, and combinations thereof.
 7. The composition of claim 1, wherein the agent comprises an ADP-ribosyltransferase.
 8. The composition of claim 1, wherein the agent comprises C2I from Clostridium botulinum toxin C2.
 9. The composition of claim 1, wherein the agent comprises a fluorescent agent.
 10. The composition of claim 1, wherein the target cell binding unit comprises the binding domain of the heavy chain of C. botulinum neurotoxin C1 (C1 Hcc), wherein the pore-forming unit comprises the pore-forming domain of Clostridium botulinum toxin C-2, and wherein the payload unit comprises C2I from Clostridium botulinum toxin C-2.
 11. The composition of claim 1, wherein the payload unit comprises a polypeptide having at least 90% sequence identity to amino acid sequence of SEQ ID NO:
 6. 12. A method of delivering an agent to a target cell, comprising administering the composition of claim 1 to a target cell.
 13. The method of claim 12, wherein the composition is administered to a subject by injection, wherein the subject comprises the target cell.
 14. The method of claim 12, wherein the target cell is a member selected from the group consisting of a cell of a brain tumor, a cell of a neuroblastoma, and a cell of a retinoblastoma, peripheral neuron; motor neuron, sensory neuron, and combination thereof.
 15. A composition for delivering an agent to a target cell, comprising a target cell binding unit, a pore-forming unit, a rigid synthetic linker linking the target cell binding unit and the pore-forming unit, and a payload unit comprising the agent, wherein said payload unit binds non-covalently to a pore formed by the pore-forming unit upon activation and oligomerization of the pore-forming unit; wherein the pore-forming unit is a polypeptide derived from a first AB type of non-neurotoxic binary toxin, and wherein the target cell binding unit is a polypeptide derived from a second AB type of toxin, said second AB type being different from the first AB type, and wherein the target cell binding unit binds to a neural cell; and wherein the rigid synthetic linker comprises a polypeptide having at least 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:
 5. 16. The composition of claim 1, wherein the neural cell is a sensory neuron.
 17. A composition for delivering an agent to a target cell, comprising a target cell binding unit, a pore-forming unit, a rigid synthetic linker linking the target cell binding unit and the pore-forming unit, and a payload unit comprising the agent, wherein said payload unit binds non-covalently to a pore formed by the pore-forming unit upon activation and oligomerization of the pore-forming unit; wherein the pore-forming unit is a polypeptide derived from a first AB type of non-neurotoxic binary toxin, and wherein the target cell binding unit is a polypeptide derived from a second AB type of toxin, said second AB type being different from the first AB type, wherein the target cell binding unit binds to a neural cell, and wherein the target cell binding unit comprises a polypeptide having at least 90% sequence identity to the amino acid sequence 230-426 of SEQ ID NO: 1, wherein the pore-forming unit comprises a polypeptide having at least 90% sequence identity to the amino acid sequence 1-591 of SEQ ID NO: 2, and wherein the rigid synthetic linker comprises a polypeptide having at least 90% sequence identity to the amino acid sequence set forth in SEQ ID NO:
 5. 